1. Technical Field
The present invention relates to a novel die casting process and to a solid lubricant composition useful as the plunger lubricant in die casting.
2. Description of the Prior Art
Die casting is a well known process in which molten metal is introduced into a shot chamber of a die casting machine The molten metal is then forced by the application of pressure into the die cavity of a mold. The application of pressure during casting produces stronger, more accurate and more consistent castings In what is known as a cold chamber process, the shot chamber is unheated except from the heat from the molten metal. A plunger is reversibly movable in the shot chamber, and when actuated, moves forward to exert pressure on the molten metal. A cold chamber process can require high pressures on the molten metal. This in turn requires close dimensional plunger tolerances which can create severe friction problems. Efficient lubrication is necessary to prevent metal adhering to the walls of the shot chamber and to the tip of the plunger and to minimize galling and wear of the plunger and shot chamber.
Most plunger lubricants which are in use today are oil or water-based emulsions of an active lubricating ingredient. They are liquid. The water-based emulsions can also contain other active lubricating ingredients such as synthetic or natural waxes, oils, polymers, graphite, esters, other metallic soaps, fatty acids and fatty acid soaps. These liquid plunger lubricants are typically sprayed into the shot chamber, dripped onto the plunger, or even brushed onto the plunger.
The use of the liquid lubricants creates many problems. One problem is the creation of smoke or steam when they encounter the hot surfaces in the shot chamber, which smoke or steam is an environmental and safety hazard. Another is that the liquid becomes deposited on the ceiling of the die casting plant, on crane rails and adjacent equipment, and on the outside of the die casting machine creating a deposit which must be cleaned up. Also, a substantial volume of the liquid lubricant must be used.
Solid lubricating compositions are known. U.S. Pat. No. 3,895,899 discloses the use of a solid lubricating composition for the manufacture of metallic parts by extrusion die forming. The solid lubricating composition comprises an inorganic binder resistant to elevated temperatures. The composition also contains either graphite or molybdenum disulfide, and optionally, an alkaline earth metal.
Similar lubricating compositions are marketed by E/M Lubricants, Inc. under the trademark EVERLUBE. The solid lubricating compositions can contain an organic binder such as a phenolic resin or an inorganic binder such as sodium silicate. The active lubricating ingredient in the compositions can be molybdenum disulfide with or without graphite.
Prior U.S. Pat. No. 4,210,259 discloses a solid epoxy coating, suitable for metal container end walls. The coating can contain 20-30% of a lubricating additive such as polyethylene, a synthetic wax such as ACRAWAX marketed by Glyco Chemicals, Inc., and carnauba wax. It is indicated in the patent that combinations of the lubricating additives can be used.
Prior U.S. Pat. No. 2,530,838 discloses a water-dispersible lubricating composition useful for drawing wire or metal stock. The lubricating composition comprises 10-40 parts of a synthetic wax having a melting point of at least about 95.degree. C. (203.degree. F.), about 35-85 parts of an inorganic boron containing compound such as a metal borate, and 5-25 parts of a water-soluble organic binder. Examples of suitable binders are polyhydric alcohols and the fatty acid esters thereof, e.g., glyceryl monostearate and diglycol stearate. The lubricating composition is dispersed in water at a temperature of at least about 90.degree. C., and then the water dispersion is applied to a surface of wire or metal stock, for instance by dipping the wire or metal stock in the solution, and dried. The stock, containing a dry film of the lubricant can then be drawn into a desired shape.
Prior U.S. Pat. No. 4,766,166 discloses a blend of a high melting synthetic wax such as a Fischer-Tropsch wax with a polyethyelene material, useful as a lubricant during the extrusion of plastics.